Packages comprising autonomous electronic and electromechanical systems that are implanted in the body of animals, such as sensors and their associated electronic circuits that function to amplify the sensor signals and transmit them to a nearby receiver, require a power source. Today, these packages are powered externally by batteries. The smallest batteries are, however, much larger than the implantable sensors and their associated signal amplifier circuits. For this reason, the size of autonomous packages that include a sensor, an amplifier-transmitter, and a power source are generally defined by the battery. Batteries cannot be made as small as the sensors or amplifiers because the batteries require cases and seals, the miniaturization of which is difficult and prohibitively expensive.
Known fuel cells are also much larger than available sensors because they require a case and a seal, and usually also a membrane, which is difficult to miniaturize and seal. By way of illustration, in a conventional fuel cell, usually a membrane separates the cathode and anode compartments of the cell so as to separate electrooxidized reactant of the anodic reaction and electroreduced reactant of the cathodic reaction. If these reactants were not separated, they could react with one another, thereby reducing the power and the efficiency of the cell. As the miniaturization and sealing of membranes present difficult manufacturing issues, conventional fuel cells are generally much larger than is desirable for certain applications.
Biological fuel cells, also known as biofuel cells, have received much attention in past years. Herein, the term “biological fuel cell” or “biofuel cell” refers to an electrochemical cell having performance attributes that permit its use as a power source for an implanted device in a biological system, such as an animal, including a human, or a plant. Biological fuel cells generate electrical energy using components found in biological systems, such as sugars, alcohols, carboxylic acids, carbohydrates, starches, cellulose, and oxygen. Such devices are generally disclosed and discussed in the above-referenced U.S. Pat. Nos. 6,294,281 and 6,531,239. Miniature biofuel cells having oxygen electroreducing cathodes and glucose electrooxidizing anodes are of current interest because such cells may power future autonomous electronic and electromechanical systems implanted in a biological system, and particularly, the human body.
Numerous biofuel cells have been described in the past fifty years. However, only a few of these biofuel cells could be operated under physiological conditions, which are the conditions relevant to operation in the body of animal. Physiological conditions include, for instance, a pH of about 7.2 to 7.4, a temperature of near 37° C., and a chloride concentration of about 0.14 M. The power density of known biofuel cells is small, the typical currents per square centimeter of electrode area being less than 1 mA/cm2, and usually less than 100 μA/cm2.
Furthermore, known biofuel cells having higher power densities require ion-conducting separation membranes, such as Nafion™ membranes. These membranes are required because components of the catalysts of the anode and cathode reactions are often dissolved or reversibly adsorbed on the electrodes. Most of these known biofuel cells comprise two different dissolved or absorbed enzymes, one in the anode compartment, and the other in the cathode compartment. The most efficient of these cells also comprise diffusional redox mediators dissolved in their respective compartments. These mediators carry electrons between the electrodes and the dissolved enzymes. Because these fuel cells have limited power densities and unstable dissolved enzymes, and require membranes that are difficult to miniaturize, they have not been produced in dimensions smaller than 1 mm in length, width, or height.
As the electronic circuits and sensors used in implantable devices have become miniaturized, autonomous sensor-amplifier-transmitter devices of millimeter and sub-millimeter dimensions can be built. For example, signal amplifier circuits and sensors with footprints under 1 mm2 can be produced. The size reduction in these implantable devices is severely limited, however, by battery size and/or cost. For example, smaller batteries, such as lithium batteries of sub-millimeter dimensions, are prohibitively expensive to manufacture. Therefore, reduction in the size of implantable medical devices depends on the availability of power sources that can be miniaturized to dimensions comparable to the implantable devices themselves, and on the cost of such power sources. These factors are important, for example, in the development of autonomous medical sensors and associated transmitters, or of receivers and associated actuators.
Accordingly, the development of a miniaturized power source is desirable. Further, the development of a power source, having millimeter to sub-millimeter dimensions, is desirable. Still further, the development of a power source that can function under physiological conditions is desirable.